The present invention relates to sensor detection signal extracting circuit, and more specifically to a circuit capable of exactly detecting the level of change in pressure by using a bridge circuit type sensor.
When a change in a physical quantity such as pressure, etc. is electrically detected by using a bridge-type sensor, a bridge circuit adjusts the balanced state with no pressure applied to the sensor, when pressure for example should be detected, and then tries to balance the output from the sensor at "0". However, due to the change in environmental temperature, it is hard for the bridge circuit to retain a balanced state even if the balanced state at "0" can be attained for a brief moment. As a result, an excess voltage is generated in the bridge circuit. Since the value of the excess voltage changes according to the voltage variation in the sensor's operational voltage source, the state of the sensor balanced at "0" cannot be retained for a long time. The unnecessary voltage generated at the output of the sensor is referred to as an "offset voltage".
Since the offset voltage is not necessary, it should be deleted, but by an effective circuit configuration.
An effective circuit should be designed to offset the above described offset voltage through feedback technology so that only the true output of the sensor, which changes according to a change in the physical quantity such as pressure, etc., can be obtained as the output of an amplifier.
FIG. 1 shows a circuit for explaining the configuration of the principle of a sensor detection signal, i.e. a sensor signal extracting circuit of the prior art technology. In FIG. 1, an appropriate voltage is applied to a sensor 1 used in the sensor detection signal extracting circuit. The sensor 1 is a bridge circuit type, and the balanced state is carefully retained as described above, though with an offset voltage generated. Therefore, the true sensor output determined depending on the change in the physical quantity such as pressure, etc. is outputted as superposed onto the offset voltage. The output of the sensor is applied to one input terminal of an adder 3. The output of the adder 3 is applied to the input terminal of amplifier G.sub.1. Then, the output of the sensor 1 is amplified by amplifier G.sub.1. The amplified signal is outputted to an output terminal Out, and applied to an integrator 4 functioning as a low-pass filter.
The integrator 4 is provided with operational amplifier A.sub.1, and the output of amplifier G.sub.1 is applied to the inverted input terminal (-) of operational amplifier A.sub.1 through resistor R.sub.1. The output of operational amplifier A.sub.1 is applied to the other input terminal of the adder 3, and fed back to the inverted input terminal (-) of operational amplifier A.sub.1 through capacitor C.sub.1. The cut-off frequency of the integrator 4 depends on the values of resistor R.sub.1 and capacitor C.sub.1. The cut-off frequency is set to a value a little lower than the minimum frequency of the frequencies of the signals contained in the true sensor output determined depending on the change in the physical quantity such as pressure, etc. so that the true output of the sensor can be separated from the offset voltage.
Capacitor C.sub.1 can be charged with a necessary voltage to eliminate the offset voltage outputted by the sensor 1. This voltage is applied to the adder 3 and added to the offset voltage outputted by the sensor 1. Hence, since this voltage applied from the integrator 4 to the adder 3 has a reversed polarity (equal in value but different in sign) to the voltage applied from the sensor 1 to the adder 3, they can be offset when added together by the adder 3, resulting in no offset voltage in the output of the adder 3. Therefore, only the true sensor output determined by the change in physical quantity such as pressure is outputted to the output terminal Out after being amplified to a predetermined level by amplifier G.sub.1.
Since the sensor 1 outputs an inevitable offset voltage, the offset voltage changes according to the change in the voltage of the sensor's operational voltage source 2. If the frequency of the change in the offset voltage is lower than the cut-off frequency of the integrator 4, then the voltage charged to capacitor C.sub.1 changes corresponding to the change in the offset voltage, thereby maintaining the capabilities of the sensor detection signal extracting circuit.
However, if the frequency of the change in the offset voltage is higher than the cut-off frequency of the integrator 4, then the change in the offset voltage can be controlled or removed by the operation of the integrator 4. Therefore, a voltage not reflecting the change in the offset voltage is applied to the adder 3 from the integrator 4. As a result, the offset voltage and the voltage outputted from the integrator 4 are not offset at the adder 3 to each other, resulting in the offset voltage remaining undesirably. Furthermore, since the residual voltage is amplified by amplifier G.sub.1, it is outputted to the output terminal Out. Therefore, a true sensor output determined depending on the change in physical quantity such as pressure, etc. cannot be outputted to the output terminal Out by the conventional sensor detection signal extracting circuit. This inconvenience occurs when the frequency of the change in the offset voltage is higher than the cut-off frequency of the integrator 4. However, if the cut-off frequency of the integrator 4 is set to an excessively high value, then the true output of the sensor cannot be separated from the offset voltage, and furthermore, unnecessary signals generated independently of the voltage change of the sensor's operational voltage source 2, such as a change in the offset voltage caused by an impact signal induced externally to the sensor 1 and the circuit, cannot be successfully removed.